Fresnel zone

About: Fresnel zone is a research topic. Over the lifetime, 2337 publications have been published within this topic receiving 37650 citations.

Full-transparent zone plates for THz focusing

Abstract: A full-transparent zone plate (FTZP), which can reuse the wave blocked in the focusing of the Fresnel zone plate (FZP), is proposed to improve the efficiency of terahertz (THz) focusing without aberration. We find that the substrate thickness of the FTZP has a great influence on the focusing intensity, which results from the Fabry–Perot effect. The focusing efficiency of FTZPs could be about twice as high as that of FZPs, but the widths of both focus spots are comparable with the wavelength. The experimental results are in good agreement with the simulation.

Differential Phase Contrast Imaging in the Scanning Transmission X-Ray Micro scope,

Abstract: : Calculations of the contrast transfer functions for differential phase contrast imaging are presented and compared to those for absorption imaging in the x-ray microscope. In the scanning transmission x-ray microscope (STXM) the specimen is scanned in a raster by an x-ray probe formed with a Fresnel zone plate. To achieve near diffraction limited resolution it is necessary to have a coherent source, even when forming an incoherent brightfield image by measuring the x-ray intensity transmitted by the object. This has so far been the only imaging mode used in the STXM and for hydrated biological specimens is well suited to soft x-ray wavelengths within the water window (2.33 to 4.36 nm) where carbon absorbs much more strongly than water. However, by the use of phase contrast rather than amplitude contrast, it is possible to form images at wavelengths where the absorption is low, resulting in lower radiation dose for the same level of contrast. Calculations have demonstrated very clearly the advantages of phase contrast imaging at wavelengths outside the water window.

Batch-processing method for super-high aspect ratio diffractive optics

Abstract: A method for fabrication of diffractive optics by batch processing is disclosed, having applicability to high resolution ultra-high aspect ratio Fresnel Zone Plates for focusing of X-rays or gamma-rays having energies up to hundreds of keV. An array of precursor forms is etched into a planar substrate. Sidewalls of the forms are smoothed to a required surface roughness. A sequence of alternating layers of different complex refractive index, for binary or higher order diffractive optics, are deposited on the precursor forms by atomic layer deposition (ALD), to provide diffractive line patterns. Thinnest layers may have nanometer thicknesses. After front surface planarization and thinning of the substrate to expose first and second surfaces of the diffractive line patterns of the diffractive optic, the height h in the propagation direction provides a designed absorption difference and/or phase shift difference between adjacent diffractive lines. Optionally, post-processing enhances mechanical, thermal, electrical and optical properties.

Imaging Synthesis in the Fresnel Approximation

Abstract: Imaging by a combined optical system is considered. The idea of a general class of optical elements, distorters, is introduced. Every element is a distorter of the optical system perturbing the field propagation, excluding the pure quadratic phase corrector. It is shown that the field propagation through the system can be analysed with the aid of a simplified equivalent system consisting of the distorters alone transferred into one space of the optical system. The conditions defining the one-space method are determined.

Propagation of ultrashort pulses through lenses.

Abstract: Ordinary optical components may behave in extraordinary ways when illuminated with ultrashort optical pulses. In the cases of lenses, only the flat lenses such as Fresnel lenses, Fresnel zone plates, and holographic lenses display such anomalies. For them, the pulse may be both elongated temporally and spread spatially unless very high focal number lenses are used.